1. Field of the Invention
The embodiments of the invention discussed herein relate to a method of manufacturing a silicon carbide semiconductor device.
2. Description of the Related Art
Silicon carbide (SiC) semiconductors are thermally, chemically, and mechanically stable; and applications in, for example, light emitting devices, high frequency devices, and power semiconductor devices (power devices) in various industries is expected. In particular, high-voltage power metal oxide semiconductor field effect transistors (MOSFET) that use a SiC semiconductor have an advantage of a lower ON resistivity as compared to high-voltage power MOSFETs that use a silicon (Si) semiconductor. Further, Schottky diodes that use a SiC semiconductor have been reported to have a lower forward voltage drop than Schottky diodes that use a silicon semiconductor.
Although a tradeoff actually exists between the ON resistivity and switching speed of a power device, power devices that use a SiC semiconductor may be able to simultaneously achieve low ON resistivity and fast switching speed. Reduction of the contact resistivity of the ohmic contact (electrical contact portion) between an electrode film and the SiC semiconductor is important in reduction of the ON resistivity of a SiC semiconductor power device. Ohmic contact resistivity is also a significant problem in achieving high speed switching in a SiC semiconductor power device. The lack of a technique to form a practical low-resistivity ohmic contact adapted to device structures and production (manufacturing) processes is one obstacle in realizing a practical power device that uses a SiC semiconductor.
A method has been proposed that is widely used to form a low resistivity ohmic contact with an n-type SiC semiconductor. The method involves adhering an electrode film onto a portion of an n-type SiC semiconductor and heat treating the formed ohmic electrode structure at about 800 to 1200 degrees C. (for example, refer to Japanese Laid-Open Patent Publication No. 2013-222823, Published Japanese-Translation of PCT Application, Publication No. 2011/115294, and Japanese Laid-Open Patent Publication No. 2013-219150). Nickel (Ni), tungsten (W), titanium (Ti), and the like are commonly known electrode materials. In particular, among ohmic contacts where nickel is used as a material of the electrode, a very promising ohmic contact achieves a practical contact resistivity on the order of 10−6 Ωcm2.
Nonetheless, when nickel is used as an electrode material, the nickel film and a portion of the SiC semiconductor react consequent to the high-temperature heat treatment and form, as an electrode film, a reaction layer (e.g., nickel silicide (NiSi) film) for which the conductivity reflects a combination of nickel, silicon, and carbon (C). Here, many of the carbon atoms liberated (diffused) from the SiC semiconductor are deposited near the surface of the electrode film and the surface of the electrode film is substantially covered by a carbon layer of the deposited carbon atoms. Consequently, adhesion between the electrode film and, for example, an aluminum (Al) film (wiring layer) stacked (formed) on the electrode film for wiring becomes poor and as a result the wiring layer may peel.
To address this problem concerning the carbon film deposited on the surface of the nickel silicide film with the generation of the nickel silicide film that becomes the electrode film, Japanese Laid-Open Patent Publication No. 2013-222823 describes removal of the carbon layer by heat treatment before formation of the wiring layer on the nickel silicide film. Published Japanese-Translation of PCT Application, Publication No. 2011/115294 and Japanese Laid-Open Patent Publication No. 2013-219150 describe suppressing the deposition of carbon atoms on the surface of the electrode film by adjusting the material composition ratio of the electrode film and generating a carbide by reacting the electrode film and the carbon atoms liberated from the SiC semiconductor when the ohmic contact of the electrode film and the SiC semiconductor is formed.